In systems supporting multicast and broadcast services, it is often required to synchronize the transmission of multiple transmitters so as to boost the signal received by multiple mobile stations. For example, in IEEE 802.16 and WiMAX systems, multiple transmitters, or access nodes, need to simultaneously and synchronously transmit the same Real Time Packet/Universal Data Protocol/Internet Protocol (RTP/UDP/IP) data for streaming video and the like. The access nodes obtain the data through a common IP multicast or broadcast group over an IP multicast network. Packets for the multicast or broadcast may arrive at the access nodes with differing delays due to non-synchronous nature of IP multicast or broadcast forwarding. Packets may also be non-synchronous because packets may be lost along the multicast forwarding path.
One possible means to accomplish synchronization for multicast or broadcast transmissions between multiple access nodes is to place a special centralized node in the path of the multicast transmissions. This special node can buffer the data to remove the variability in the delay, and based on the packets that have arrived, decide on a schedule for transmission for each transmitter access node. The special node can then multicast or broadcast the data to be transmitted along with special information indicating the transmission time of each unit of data to thereby ensure synchronous transmissions of the data from the access nodes.
Such a special node is impractical because of the added expense of the node. In addition, a special node raises reliability concerns as a single point of failure within a network. It is also raises difficulty in determining a desirable location for the special node because it should be within a suitable distance from each of the access nodes to ensure synchronization between the nodes and without adding additional delays for the transmission between the special node and the multiple access nodes.
In an alternative solution to the synchronization issues, a method has been developed that ensures that all transmitters that are transmitting at a given instant are synchronized with each other even in the presence of random packet losses in the IP multicast delivery. According to this method, there is a set of packets that have arrived at each of the access nodes buffers at any given point in time. If a packet is lost or inordinately delayed along the multicast path delivering the packet to one of the access nodes, there will be a mismatch between the sequences of packets that have arrived at this node as compared to other nodes. To detect such mismatches, each transmitter node sends feedback messages identifying the packets currently in its buffer to a leader node. Each packet is identified using some unique identifier like an RTP sequence number together with a UDP checksum or the like. The length and position of the packet in the queue is also indicated.
The leader node “merges” the feedback to produce a unified or merged sequence of packets that should be transmitted by all the transmitters, and also decides the time at which transmission of these packets should begin. The leader node then sends to the access nodes the merged sequence along with the scheduled transmission time. It is assumed that the transmission parameters of the packets on the wireless interface are known a priori to the transmitter nodes. Thus, once the length of the packet is known, the duration of transmission of the packet is also uniquely known. Each transmitter node now transmits those packets out of the merged sequence that are present in its buffer at the scheduled times, and stays silent during the durations of transmissions of packets that it has not received so as to avoid causing interference. This alternative method of synchronization raises concerns based on the amount of signaling required between the leader node and the other access nodes in order to ensure synchronous transmission. In other words, this prior art solution, whilst avoiding the need for a special node, is adding another element that can cause synchronization issues.
In view of the foregoing, there is a need for a method and apparatus that enables synchronous transmission of multicast or broadcast data from access nodes that may receive the data asynchronously from a source without the addition of special network equipment and that avoids excessive signaling between the access nodes for achieving synchronous transmission.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.